Hi,
The last 3 times I purchased through Monzo I paid more money that Google says. I’ve just made a payment of AU$96.14. According to Google the conversion is £53.91; however, £54.23 were deducted from my account. I’d like to know if someone else is experimentig similar issues.
Monzo doesn’t use Google rates.
The rate to check is the MasterCard rate for the day the transaction is settled (usually a few days after the transaction date).
The Mastercard rate is a live fluctuating rate. Changes every second.
The price you’ll be charged is the Mastercard rate at the point the merchant claims the money and settles hence why it fluctuates somewhat. Google uses a static rate with a once or twice daily update if I recall correctly.
You’ve been unlucky the AUS dollar has been super weak recently, but strengthened against the pound middle of last week.
See:
If you check the MasterCard Currency Conversion Calculator
AU$96.14 would have charged you £54.57 yesterday at the settled rate.
AU$96.14 would charge you £53.98 right now (5:59am) at the unsettled rate but this may change when settled.
It can move throughout the day.
https://www.mastercard.co.uk/en-gb/consumers/get-support/convert-currency.html
I believe Mastercard have one rate per day for settlements.
It’s worth noting that Google’s currency conversations should always be taken as advisory. 
Thanks for this Rika.
Does that mean currency conversion is separate to settlements? If not, how are Revolut doing it?
Yup 
We convert it to GBP based on the rate at authorisation, at settlement there will be a different rate.
This makes chargebacks interesting as they have to be converted back to the currency of presentment at the daily rate…!